Carbon fibers grown in a vapor phase which may be said to be vapor-grown carbon fibers can be prepared by subjecting a carbon compound to pyrolysis at temperature ranging from 800.degree. C. to 1,300.degree. C. in the presence of a catalyst comprising iron or nickel in the superfine particle form. The resulting vapor-grown carbon fibers then can readily be converted into carbon fibers having a graphite structure by heating the vapor-grown carbon fibers. For example, when the vapor-grown carbon fibers are heated at temperature higher than 2,800.degree. C. to form vapor-grown and graphitized carbon fibers, crystal lattice planes of graphitized carbon fibers, having little or no defects in crystalline order, grow in the direction parallel to the fiber axis. Hence, the carbon fibers grown in the vapor phase and then graphitized can offer the characteristics that they have a high degree of physical strength and elastic modulus and that they are highly conductive thermally and electrically.
It is thus greatly expected that the vapor-grown and graphitized carbon fibers can provide composite materials having excellent properties. With this expectation, extensive review has been made with the attempt to develop molded members containing such vapor-grown and graphitized carbon fibers at a high packing density and consisting substantially solely of such carbon fibers.
As a result of such extensive review, however, it has been found difficult to form such molded members consisting substantially solely of such vapor-grown and graphitized carbon fibers and containing them at a high packing density. This difficulty may be assumed to reside in the matter that, when the vapor-grown and graphitized carbon fibers are mixed with another material to thereby form a composite material, there are employed the vapor-grown and graphitized carbon fibers that are prepared by adjusting the vapor-grown carbon fibers with a means such as a ball mill or the like so as to have an appropriate fiber length, for example, having an aspect ratio of 100 or lower in order to prevent the carbon fibers from uneven dispersion, and then by subjecting the resulting carbon fibers to graphitization. The vapor-grown and graphitized carbon fibers so prepared have a regular crystalline structure having little defect so that they are less wettable with other materials, and are high in elastic modulus. Hence, the shape of molded member of vapor-grown and graphitized carbon fibers cannot be sustained when molded members are to be formed substantially solely with vapor-grown and graphitized carbon fibers for its high packing density. Further, the resulting molded members become very fragile even if the shapes of the vapor-grown and graphitized carbon fibers could be sustained.
On the other hand, extensive review has recently been made with the attempt to develop a lithium ion secondary battery that uses carbon as an active substance for an electrode.
Although a lithium secondary battery has drawn attention due to its high energy density, there is the risk that may be caused to occur if its electrode is not sealed in a complete way because metallic lithium that is highly active to oxygen and moisture is employed for the electrode. Further, it can suffer from the disadvantage that the electrodes are short-circuited due to the formation of needle-like crystals of lithium, i.e. dendrites, on the surface of the lithium electrode. Hence, a growing interest has been shifted to the lithium ion secondary battery which uses as the electrode a carbon that can form an intercalation compound with the lithium ions, because such a lithium ion secondary battery does not cause those risks and disadvantages, even if the lithium ion secondary battery should sacrifice some degree of energy density.
Among the carbon-lithium ion intercalation compounds, the compound that contains a largest amount of lithium ions is a compound of first stage and the ratio of carbon atoms to lithium atoms is six to one (6:1). If all the carbons are used for producing intercalation compounds and all the lithium ion intercalated on a charging process are fully de-intercalated on the discharging, the quantity of electricity that can be discharged is a maximum of 372 mA hour per gram of carbon. Although a variety of efforts have been made so far to attain the theoretical quantity of dischargeable electricity, no compound of first stage having a satisfactorily high quantity of electricity discharge capability has yet been found.